1. Field of the Invention
The present invention relates to a method for preventing corrosion of a metallic layer of a semiconductor chip, and more particularly, to a method for preventing recurrent corrosion of an aluminum-containing metallic layer of a semiconductor chip caused by residual chlorine atoms.
2. Description of the Prior Art
In semiconductor processing, aluminum is the conductive material prevalently used due to its high conductivity, low cost, ease of deposition and etching, and its strong adhesion to S.sub.i O.sub.2. In almost all semiconductor chips the inter-element runners are made of aluminum. Integration of internal elements of the semiconductor chip continues to become more dense and the connection pathways between elements are required to be finer. Therefore, using pure aluminum has the following shortcomings:
(1) There is a certain solid solubility factor between silicon and aluminum such that the inter-diffusion between the two types of atoms in the semiconductor chip becomes very pronounced once the processing temperature exceeds 400.degree. C. causing a spiking phenomenon to occur. This leads to problems in the contact between the aluminum runner and the MOS element. Short-circuiting may then occur. PA1 (2) Most aluminum runners in a semiconductor chip have a poly-cyrstalline structure making them highly susceptible to conductive current which causes aluminum atoms in the runner to move along the grain boundary. This is known as electro-migration and, if severe, may result in complete opening of the aluminum runner. PA1 (1) removing the photo resistance layer on top of the metallic layer by ashing at temperatures between 178.degree. C. and 200.degree. C. after a trench etching process; PA1 (2) using an acidic solution comprising hydroxylamine (NH.sub.2 OH), hydroquinone C.sub.6 H.sub.4 (OH).sub.2, monoethanolanine (HOCH.sub.2 CH.sub.2 NH.sub.2) and water to wash off residues on the surface of the semiconductor chip; and PA1 (3) heating the semiconductor chip for a predetermined time period at temperatures between 200.degree. C. and 250.degree. C. so as to completely dissipate the chlorine atoms resided on the side walls of the metallic layer for preventing recurrent corrosion of an aluminum-containing metallic layer caused by chlorine atoms residual.
Hence, an Al-Si-Cu alloy containing 1% Si and 0.5-4.0% Cu by weight is popularly used in the processing of very large scale integration circuits. This effectively prevents the above mentioned problem from occurring.
However, regardless of whether pure aluminum or an Al-Si-Cu alloy is used, there is still the possibility of corrosion of the aluminum constituent caused by residual chlorine atoms. Please refer to FIG. 1 and FIG. 2. FIG. 1 is the chemical reaction formula of a reactive ion etching of an aluminum-containing metallic layer with chloride and chlorine (Cl.sub.2). FIG. 2 is the chemical reaction formulas of the recurrent corrosion of the aluminum constituent shown in formula I in FIG. 1. Currently, a reactive ion etching (RIE) process is used in semiconductor processing on an aluminum-containing metallic layer. The process uses a mixed gas with chloride (such as SiCl.sub.4, BCl.sub.3 and CCl.sub.4) and chlorine. As the formula in FIG. 1 illustrates, Al is converted into volatile aluminum chloride (AlCl.sub.3) in the etching of the aluminum-containing metallic layer. AlCl.sub.3 is then evacuated. Often after etching, the surface of the aluminum-containing metallic layer still contains residual AlCl.sub.3 or chlorine. The residual chlorine atoms will interact with the aluminum atoms in the aluminum-containing metallic layer to produce anhydrous aluminum chloride (AlCl.sub.3), white or yellowish hexagonal crystals formed on the surface of the aluminum-containing metallic layer. If the environment is damp anhydrous aluminum chloride (AlCl.sub.3) will interact with water molecules to generate HCl according to chemical reaction formulas II and III shown in FIG. 2, then HCl will react with the aluminum-containing metallic layer, according to the chemical reaction formula IV shown in FIG. 2 to generate anhydrous aluminum chloride (AlCl.sub.3). This causes corrosion of the aluminum-containing metallic layer.
In the RIE process above, the chemical reaction of chlorine and aluminum (formula I) on the aluminum-containing metallic layer 14 is isotropic, so when etching the aluminum-containing metallic layer 14, chloride and chlorine gas (such as a mixed gas of BCl.sub.3 and Cl.sub.2) is necessary to enhance the anisotropic etching ability of the plasma on the aluminum-containing metallic layer 14. Also, in this RIE process, a small amount of carbon tetrafluoride (CF.sub.4) or fluoroform (CHF.sub.3) is added into the plasma of BCl.sub.3 and Cl.sub.2 in order to form a polymer film to cover the surface of the side-wall of the aluminum-containing metallic layer to reduce the interaction between chlorine atoms and aluminum atoms on the side-wall of the aluminum-containing metallic layer following etching. This further improves the anisotropic etching of the plasma of BCl.sub.3 and Cl.sub.2 on the aluminum-containing metallic layer.
These methods of improving anisotropic etching on the aluminum-containing metallic layer causes an increase in chlorine atom-containing residues left on the aluminum-containing metallic layer. During etching of the side-wall of the aluminum-containing metallic layer or the polymer film, a small amount of AlCl.sub.3 or chlorine-containing polymer molecules easily adheres to the surface of the etched side-wall of the aluminum-containing metallic layer. This causes recurrent corrosion of the aluminum-containing metallic layer.